This invention relates generally to RF input circuits and particularly to an RF input circuit for use in a double conversion tuner for digital television signals such as that in the proposed Zenith/AT&T Digital Spectrum Compatible High Definition Television System (DSC/HDTV). U.S. Pat. No. 4,435,841, assigned to Zenith Electronics Corporation, is concerned with reducing second and third order intermodulation distortion in a double conversion tuning system. Essentially a series of capacitors and inductors is selectively switched to exhibit high pass and low pass characteristics over the tuning band for controlling intermodulation distortion by restricting the bandwidth of incoming signals as the various channels are tuned. U.S. Pat. No. 4,571,560, assigned to Zenith Electronics Corporation discloses a switched bandpass filter arrangement for a double conversion tuner. Each bandpass filter section includes a switching diode rendering the filter section active for passing, or inactive for suppressing, the band of frequencies to which it is tuned. Portions of the inactive bandpass filter sections contribute to developing frequency responses with notches at frequencies outside of the pass band of the active filter section. Thus some elements of the inactive bandpass filter sections are used with the active filter sections for improving the band rejection slopes. U.S. Pat. No. 5,311,318, assigned to Zenith Electronics Corporation discloses a double conversion tuner in which the first local oscillator is controlled by a low noise phase locked loop supplied with a first digital number and the second local oscillator is controlled by a second digital number that compensates the frequency of the second local oscillator for any deviation of the first IF frequency from a predetermined value. These patents provide the relevant background for the present invention.
With the advent of digital transmission systems for television signals and HDTV there is a need to improve the television tuner performance beyond that which is currently acceptable with NTSC transmissions. in particular, the tuner selectivity must be addressed. It is desirable to widen and improve the frequency response within a 6 MHz TV channel and to minimize variations in frequency response from channel to channel. It is also expected that HDTV transmission will be capable of utilizing the so-called taboo channels. With standard single conversion varactor tuned TV tuners, the above requirements are in opposition. A double conversion tuner (used for years by the cable TV industry) can yield some desired improvements but has other problems related to its wide band RF input. Unlike a single conversion tuner that has a low IF frequency which allows, to some extent, RF and local oscillator tracking, a state of the art double conversion tuner has a relatively high first IF frequency (about 1 GHz). Recent developments in dedicated microprocessors, D/A converters, electronic switching circuits and circuit integration make it possible to provide a double conversion tuner with a microprocessor controlled front end for RF selectivity, which is the subject of this invention.